The present invention relates to an improved combination of a premixing chamber and a combustion chamber, with low emission of pollutants, for gas turbines running on liquid and/or gas fuel.
As is known, a gas turbine is a machine consisting of a compressor and a turbine with one or more stages, in which these components are interconnected by a rotating shaft and in which a combustion chamber is provided between the compressor and the turbine.
Air from the external environment is supplied to the compressor for pressurization.
The pressurized air passes through a duct, terminating in a converging portion, into which a set of injectors supplies fuel which is mixed with the air to form a fuel-air mix for combustion.
The fuel required for the combustion process, which is designed to cause an increase in temperature and enthalpy of the gas, is supplied from a pressurized network and is introduced into the combustion chamber by means of one or more injectors.
A parallel fuel supply system, for generating pilot flames in the proximity of the outlet of the mixing duct, is also generally provided, in order to improve the stability characteristics of the flame.
Finally, the gas at high temperature and high pressure passes through suitable ducts to reach the various stages of the turbine, which converts the enthalpy of the gas to mechanical energy available for a user.
It is well known that, in the design of combustion chambers for gas turbines, the overriding considerations are those of flame stability and control of the excess air, the aim being to establish ideal conditions for the combustion process and minimize the output of pollutants. 
A second element influencing the design of combustion chambers of gas turbines is the tendency to make combustion take place as near as possible to the dome of the combustion chamber.
More specifically, the prior art provides a combination of the premixing type, in other words one in which a premixing chamber is used and is located upstream from the combustion chamber and is separated from the latter by a constriction.
Both this premixing chamber and the combustion chamber are surrounded by a cavity containing pressurized air circulating in the opposite direction to the flow of combustion products leaving this combustion chamber.
This air is used as the combustion air to be mixed with the fuel in the premixing chamber and as the air for cooling both the combustion chamber and the combustion products.
Additionally, in the combination described above, the flow of the combustion air from the cavity to the premixing chamber, through apertures provided in the outer surface of the latter, can be constricted, in order to keep emissions of nitrogen oxide pollutants low at all levels of loading of the turbine.
The constriction is applied as a function of the quantity of fuel used, so that the ratio between the combustion air and the fuel constant is kept at the optimal value.
To prevent the flame from being extinguished or becoming unstable in any way, an annular set of holes is provided for an additional injection of fuel, so that the combustion region immediately downstream from the constriction is enriched with fuel.
In the prior art, one or more injectors are used, to enable liquid fuel to be used as an alternative to gas fuel or in a complementary way; these injectors produce diffusion flames and, owing to the lack of fuel and air mixing, cause high concentrations of polluting emissions. 
At the present time, there is an increasing market demand for gas turbines which can run equally well on gas fuel and/or liquid fuel.
This flexibility must be provided without prejudice to the primary aim of reducing polluting emissions to a minimum, while also meeting other requirements for satisfactory combustion.
The object of the present invention is therefore to overcome the aforementioned drawbacks, and particularly to provide an improved combination of a premixing chamber and a combustion chamber for gas turbines running on liquid and/or gas fuel, which ensures a low emission of pollutants.
Another object of the present invention is to provide an improved combination of a premixing chamber and a combustion chamber, with low emission of pollutants, for gas turbines running on liquid and/or gas fuel which also provides good flame stability and reduces the oscillations of pressure in the combustion chamber.
Yet another object of the present invention is to provide an improved combination of a premixing chamber and a combustion chamber, with low emission of pollutants, for gas turbines running on liquid and/or gas fuel which provides a high combustion efficiency.
An additional object of the present invention is to provide an improved combination of a premixing chamber and a combustion chamber, with low emission of pollutants, for gas turbines running on liquid and/or gas fuel which can increase the average life of components subjected to high temperatures.
A further additional object of the present invention is to provide an improved combination of a premixing chamber and a combustion chamber, with low emission of pollutants, for gas turbines running on liquid and/or gas fuel which is particularly reliable, simple and functional, and whose production and maintenance costs are relatively low. 
Advantageously, an improved combination of a premixing chamber and a combustion chamber, with low emission of pollutants, for gas turbines running on liquid and/or gas fuel according to the present invention can be made so that it is fairly easily interchangeable with combustion chambers according to the prior art which have already been installed.